Nowadays, mobile electronic devices, such as cellular phones, are quickly becoming something essential for our lives. It is estimated that the demand of cellular phone will exceed 1.2 billion at 2010, and it will increase to about 1.5 billion after 2012. By that time, there will be more than 5% of the cellular phone sold at 2012 that are configured with miniaturized projection modules, i.e. there will be about 75 million cellular phones that have miniaturized projection modules built therein. Nevertheless, it is noted that the key element for any miniaturized projection modules is the micro scanning mirror, as it can be the key component determining how a miniaturized projection module can be embedded inside a portable electronic device, such as a cellular phone a digital camera a notebook computer a person digital assistant (PDA), or a portable game console, and so on. In addition, the miniaturized projection module itself is the key component for electronic industry for developing products such as laser projectors, laser TVs, head-mount display modules, barcode readers, optical communication switches, and head-up displays, etc.
Depending on the type of light source used, the miniaturized projection modules can be divided into two categories, which are laser-based projection modules and LED-based projection modules. Nevertheless, there are significant differences between this two types of projection modules, which are described as following:                (1) By the use of laser light source of comparatively wider color gamut, the laser-based projection modules are enabled to produce images with better color saturation and higher resolution.        (2) Being the core structure of the laser-based projection modules, the MEMS double-axis reflection mirror are structured simply as a unique single mirror; and comparatively, since the LED-based projection modules adopt DLP technology, there are more than millions of micromirrors to be integrated into one single component. In addition, as the double-axis reflection mirror can be produced in a batch process manner by the use of a MEMS semiconductor process, the production of the laser-based projection modules are featured by its high production yield and low manufacturing cost.        (3) The laser-based projection modules are capable of producing high brightness, high directional projection upon any surface, no matter it is flat or curved, and without the use of any complex optical lens modules for focusing, by that the laser-based projection modules can be miniaturized so as to be embedded in electronic devices that are becoming thinner, smaller and lighter. Therefore, the laser-based projection modules are commercially advantageous over the LED-based projection modules in every aspect including size, cost and performance.        
Based upon the actuation principle, the MEMS scanning mirrors, being the key component of the laser miniaturized projection module, can be divided into the following types: the electromagnetic type, the electrostatic type, the piezoelectric type, and the thermoelectric type, etc. Nevertheless, each of those different types of MEMS scanning mirrors has its shortcomings that required to be overcome. For the electrostatic MEMS scanning mirrors, the micromirrors actuated by electrostatic forces may cause a problem of insufficient scanning angle when they are operating in an off-resonance state, and are usually required to be applied by a voltage higher than 80V for driving the same, not to mention that not only the components used in electrostatic MEMS scanning mirrors are prone be damaged by absorption effect, but also the manufacturing yield of the electrostatic MEMS scanning mirrors can be unsatisfactory since the structure of the electrostatic MEMS scanning mirrors may be very complicated. For the electromagnetic MEMS scanning mirrors, as there are usually coils formed on the micromirrors by electroplating, the micromirrors can be deformed by the heat accumulation resulting from the coils. For the thermoelectric MEMS scanning mirror, it is impractical by its low scanning frequency that is resulted from the problem of thermal effect. For the piezoelectric MEMS scanning mirrors, not to mention that it can be very large in size, the comparatively low displacement resulting from the piezoelectric actuation may cause a problem of insufficient scanning angle. Thus, for designing a good portable laser miniaturized projection module, it is importance to feature out an optimal way for actuating the MEMS scanning mirrors.
Currently, the most popular and successful design used in the miniaturized projection module for driving the scanning components is the electromagnetic actuation design, in which there are coils formed on the micromirrors by electroplating so as to be used for generating Lorentz forces that are used as the driving force for driving the micromirrors to move. Nevertheless, although the electromagnetic miniaturized projection module has advantages including large scanning angle, low working voltage, linear stepping angle effect, and high resolution, etc., it is disadvantageous in that: manufacturing yield of the electrostatic MEMS scanning mirrors can be unsatisfactory since the structure of the coils formed on the micromirrors may be very complicated that can only be achieved after tens of photo mask processes; and also, the micromirrors can be deformed by the heat accumulation resulting from the charged coils. Thus, the manufacturers of miniaturized projection module are now focusing their researches for resolving the aforesaid problems.
It is noted that the Lorentz forces used for driving the electromagnetic scanning components are generated by the interactions between currents charging in the coils that are formed on the scanning components and magnetic fields resulting from the permanent magnets that are disposed at two opposite sides of the scanning components. One such electromagnetic scanning component is exemplified in U.S. Pat. Pub. No. 20050253055, entitled “MEMS device having simplified driver”, in which both the fast scan axis and the slow scan axis of any silicon-based mirror are electroplated with metal coils for enabling the two axes to be driven by Lorentz forces. However, although by the means disclosed in the aforesaid U.S. patent application the scanning angle is increased, the mirror deformation caused by the heat accumulation from the charged coils is still unresolved.
One example relating to the use of magnetostatic forces for driving micromirrors is disclosed in U.S. Pat. No. 6,965,177, entitled “Pulse drive of resonant MEMS drivers”, in which a mirror component are configured with permanent magnets attached respectively to the edges of its two axes while there are several electromagnetic coils being disposed underneath the same that are connected to an AC power source and thus are capable of generating a magnetic field varying with the AC frequency of the AC power source, so that by the interaction between the magnetic field and the permanent magnet, the mirror component are enabled to perform resonant oscillations according to the AC frequency. However, the aforesaid means is disadvantageous in that: since it is difficult to attach magnets on the mirror component and consequently the assemble cost is increased, mass production is infeasible, not to mention that the attaching of the magnets on the micromirrors that can be very fragile is going to have adverse effect upon the resonant frequency of the micromirrors and the durability of the same as well.
In addition, another example of a conventional electromagnetic driving device that is driven by Lorentz forces caused by charged coils is disclosed in U.S. Pat. Pub. No. 20070047046, entitled “Micro-mirror device and array thereof”, in which a micro mirror device is structured as a dual-ring structure having a mirror plate sandwiched between an inner ring and an outer ring while enabling the mirror plate to be coupled to the inner ring and the outer ring by interconnecting reinforcement rims. the disposing of the inner ring is simply for stabilizing the mirror plate, preventing the mirror plate to move relative to the inner ring, and thereby, enabling the mirror plate to vibrate in synchronization with the inner ring without causing any unwanted angle amplification effect. However, it is noted that the additional inner ring not only will cost the size of the mirror device to increase, but also it has very little effect for stabilizing the mirror plate.
Accordingly, the conventional optical scanning device, no matter it is used for optical detection or laser projection is primarily composed of: a scanning mirror located at the center thereof; a single-ring structure arranged surrounding the scanning mirror; and a frame. Moreover, as the aforesaid three components are usually coupled to each other by two sets of torsion beams that are arranged orthogonal to each other, not only the scanning angle can not be enlarged effectively, but also the projection images can be distorted since the scanning mirror might be deformed by the high temperature of the coils. Moreover, as in most electromagnetic driving devices the coils that are formed by electroplating are formed as a double-layered structure, manufacturing yield can be unsatisfactory and the manufacturing cost may be high also since the structure of the double-layered coils may be very complicated that can only be achieved after tens of photo mask processes.